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http://pubs.acs.org/doi/pdf/10.1021/acscatal.6b03253

Single Electron Transfer Steps in Water Oxidation
Catalysis. Redefining the Mechanistic Scenario
Ignacio Funes-Ardoiz†, Pablo Garrido-Barros†,‡, Antoni Llobet†,§,* and Feliu Maseras†,§,*
† Institute of Chemical Research of Catalonia (ICIQ), Avgda. Països Catalans, 16, 43007
Tarragona, Spain
‡ Departament de Química Física i Inorgànica, Universitat Rovira i Virgili, Campus Sescelades.
C/ Marcel.lí Domingo, s/n, 43007, Tarragona, Spain.
§ Departament de Química, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
KEYWORDS: water oxidation, water splitting, first row transition metal complexes, DFT,
mechanism.

ABSTRACT: The systematic computational study of the mechanism for water oxidation in four
different complexes confirms the existence of an alternative mechanism to those previously
reported: the single electron transfer - water nucleophilic attack (SET-WNA). The new

1

mechanism relies on two SET steps, and features the existence of an intermediate with a
(HO•••OH)-moiety attached to the metal center. It is operative in at least three representative
copper based complexes, and is the only option that explains the experimentally observed
efficiency in two of them. The proposal of this new reaction pathway redefines the mechanistic
scenario and importantly, generates a completely new avenue for designing more efficient water
oxidation catalysts based on first row transition metals.

Introduction
Water splitting driven by sunlight to produce molecular oxygen and hydrogen is regarded as
one of the most promising approaches for the generation of clean fuels in a sustainable manner.1,2
Hydrogen generated in this manner is generally labeled as solar-hydrogen3 and is regarded as a
way of storing solar energy into chemical bonds, in similar manner as done by the photosystem
II of green plants and algae.4
From an electrochemical perspective the water splitting reaction consists in two half reactions:
water oxidation to molecular oxygen and proton reduction to hydrogen. Particularly the water
oxidation to dioxygen has been traditionally regarded as the bottleneck for the design of practical
devices that can carry out water splitting with sunlight.5-8
The water oxidation to dioxygen reaction, besides being energy demanding (Eo = 1.23 V vs
NHE at pH = 0), is also molecularly complex since requires the break of four H-O bonds, the
releasing 4 electrons and 4 protons together with the formation of an O-O bond. This complex
mechanistic scenario generally translates into very high over-potentials needed for the reaction to
proceed.9

2

One of the potential strategies to overcome high activation energies consists in the
involvement of transition metals as catalysts. Indeed a number of Ru10-17 and Ir18-22 complexes
have been recently described as efficient catalysts for this reaction. Furthermore the
understanding of the different mechanisms involved in these catalytic processes as well as the
potential deactivation pathways, has been crucial for the development of the field. However
better catalysts, more robust and efficient, are needed to be able to incorporate them in devices
that can carry water splitting with sunlight.
In order to progress in this front it is essential to gain a deeper understanding of the reaction
mechanisms that can operate in water oxidation catalysis. From this perspective it is imperative
to spectroscopically characterize reaction intermediates as well as their reactivity. In this respect
the theoretical methodologies become an extremely valuable tool to complement experimental
work, especially in systems with such a complexity as the water oxidation reaction catalyzed by
transition metals.
Mechanistic and theoretical studies have been carried out mainly with Ru complexes and have
led to propose two main pathways for the O-O bond formation step depending on whether an
external water molecule is involved or not. Thus a water nucleophilic attack (WNA) and
interaction of two M-O species (I2M) have been extensively discussed by our group and
others.23,24
Transition metals in their high oxidation states containing the M-O group can be described
using two resonant forms depending on whether the oxidation occurs solely at the metal center or
at the oxygen, as depicted on the left hand side of Figure 1. All over the present paper we will

3

use formal oxidation states since this allows easily tracking electron trafficking, although it is
obvious that the real species will be a mixing of both resonant forms.
A WNA mechanism (Figure 1) is found when the auxiliary ligands favor the stabilization of the
oxo form shown on Fig. 1 top and an I2M mechanism is found when the favored species
resemble those of the oxyl radical form depicted in the lower part of Figure 1. The WNA
involves a concerted two electron process from the incoming water molecule to the metal center
of the M-oxo group resulting in the formation of a peroxide intermediate and the reduction of the
metal center by two units. On the other hand the I2M mechanism involves a radical coupling
where the oxidation state of the metal center remains unchanged.

Figure 1. Schematic view of Water Nucleophilic Attack (WNA) and Interaction of Two M-O
Species (I2M) mechanisms. Formal oxidation states for the metal and oxygen atoms are
indicated as superscript.
Recently several first row transition metal complexes have been reported as catalysts for the
water oxidation reaction.25-40 While these catalysts are of interest because of their high
abundance and low toxicity, their performance is much poorer than their Ru or Ir analogues, and
in addition their mechanistic pathways are in most cases basically unknown.28-30,41-43

4

We have very recently reported a new complex based on Cu, containing the amidate ligand
OPBAN that can carry out the water oxidation reaction in a very efficient manner. Surprisingly
from a mechanistic perspective it does not follow the WNA or I2M schemes just described but
rather a step by step one electron process that we have termed SET-WNA. Incidentally in
parallel there is an increasing recognition of the presence of single electron transfer (SET) steps
in first-row homogeneous catalysis.45,46
Therefore, it seems necessary and reasonable to raise these approaches to water oxidation
catalysis using first row transition metals with the final aim of proposing a reasonable and
complete mechanistic scenario.
The present woks thus aims at the description of a complete and integrated view of all the
potential pathways leading to low energy O-O bond formation by transition metal catalysts.
Herein, we present a comprehensive DFT study on the water oxidation mechanism, with special
attention to emerging copper catalysis that allows us to map the different accessible O-O bond
formation pathways that in turn reveal the most important features for WO catalyst design.

Results and Discussion
1. The SET-WNA mechanism in copper systems: [(OPBAN)CuII]2-, [(6,6’dhbp)CuII(OH2)2]2-, [(bpy)CuII(OH)2]
We first proposed the existence of a single electron transfer - water nucleophilic attack (SETWNA) mechanism in a joint experimental and computational communication on
[(OPBAN)CuII]2- (OPBAN= o-phenylenebis-(oxamidate)) complex, hereafter [(L1)CuII]2-, 1a.31
We will briefly recall and extend the results on this system here, and compare them with those on

5

other copper complexes that have been shown experimentally to efficiently catalyze water
oxidation. The new complexes studied are [(6,6’-dhbp)CuII(OH2)2]2-, (6,6’-dhbp= [2,2'bipyridine]-6,6'-bis(olate)), hereafter [(L2)CuII(OH2)2], 2a,30 and [(bpy)CuII(OH)2] (bpy= 2,2'bipyridine), hereafter [(L3)CuII(OH)2], 3a.28
The key intermediates computed oxidation sequence for the three complexes is shown in eqs. 1
to 3 below, respectively (the full set of accessible species is reported in the Supporting
Information, Supplementary Figs. 1 and 2).

(1)

(2)

(3)
The active species is formed by two consecutive one electron oxidations that can be metal or
ligand based. The highest potentials correspond to system 3, where the L3 ligand is not oxidized
since its redox potential is too high and thus the second electron is removed directly from the CuO moiety, forming an oxyl group, 3c. In the other two systems the ancillary ligands acts as redox
non-innocent. For 2, the first step involves the removal of an electron from the ligand whereas
this happens in the second step for complex 1.
The set-up for the reaction between species 1c, 2c, 3c with a hydroxyl/oxyl group bound to the
metal and an external hydroxyl group, is typical of the WNA mechanism. But we did not find
this mechanism to be preferred in any of the three cases. For the 1c/2c complexes, we could not

6

locate a transition state connecting them directly to the resulting intermediates 1e/2e (see Figure
2), where both oxygen atoms have a formal oxidation state of -1. The reason for this
impossibility is apparent in the potential energy scans in the top part of Figure 2.
Intermediates 1c/2c and 1e/2e are at the bottom of two wells in red in the potential energy
scan, and the energy of both curves increase sharply before they meet. Instead, the connection
between the two intermediates takes place through an additional intermediate 1d/2d, depicted in
blue. This intermediate has a complex electronic structure as the unpaired electron is shared by
both oxygen atoms, forming a 2c-3e- bond47 with a length around 2.30 Å. This bond has a formal
order of 0.5 and a formal oxidation state of -1.5 in each oxygen atom (Figure S3).

7

Figure 2. (Left) Free energy profiles of [(L1)CuII]2-(top) and [(L2)CuII(OH2)2] (bottom) where
L1 = o-phenylenebis(oxamidate) and H2L2 = [2,2'-bipyridine]-6,6'-bis(olate)). Energies in
kcal/mol. (Right) Potential energy relaxed scan of both complexes of the O-O reaction
coordinate. The draw at right-top is based on one reported in ref. 31.

8

Intermediate 1d/2d connects 1c/2c to 1e/2e through single electron transfer steps. The
connection between 1c and 1d takes place through outer-sphere transfer, as the O-O distance is
above 3.5 Å. There is no transition state in the potential energy surface for this step, but we could
estimate a free energy barrier of 5.5 / 5.6 kcal/mol as the difference between a long-range adduct
and the separate reactants. It is worth noticing that there is no crossing between the curves
corresponding to 2c and 2d in the potential energy surface, but this has no significant effect on
the reactivity, as the free energy of the separate reactants associated to 2c becomes lower after
introducing the entropic contributions. For the connection between 1d and 1e, we could locate
the transition state TS 1d-e (Figure 3).

Figure 2. Displacement vectors of the reaction coordinate normal mode of TS 1d-e.
It contains an unpaired electron on the ligand (spin 1.0) and an additional open-shell between
the copper (spin 0.6) and oxygen (spin -0.3 each) centers. This transition state has an energy 9.6
kcal/mol above intermediate 1d. This constitutes the highest barrier in the process, which is thus
obviously affordable at room temperature. Although the HO•••OH moiety is not attached
covalently to the metal center (only by hydrogen bonds), the normal mode vectors show clearly
the participation of the catalyst in the transition state. The connection between 2d (quartet) and

9

2e(doublet) should take place through a minimum energy crossing point (MECP) which we
could not locate for technical reasons, but we could estimate for it a low relative energy of 2.9
kcal/mol from the relaxed potential energy scan. The existence of states with different
multiplicities can be in fact related to the concept of multi-state reactivity.46 In spite of the minor
differences between systems 1 and 2, it is clear from Figure 2 that they share the same
mechanism. We notice here that the possible intramolecular O-O bond formation process
between the two hydroxyl ligands in 2c was found to have a prohibitively high barrier (Figure
S4).
A slightly different scenario is obtained for complex 3c, where the ligand oxidation is very
high in energy and thus is not oxidized. In this case, we could characterize an intramolecular
WNA mechanism for O-O formation. The reductive coupling between the hydroxyl and the oxyl
center can take place through a transition state TS 3c-d' that is 7.0 kcal/mol above 3c (Figure
S5). In this transition state, the O-O bond formation occurs simultaneously with the reduction of
CuIII to CuII. However the SET-WNA pathway involving an external hydroxyl group gives a
lower barrier (Figure S6). Indeed, there is a 2c-3e- O---O intermediate similar to those reported
above, which evolves through a low energy path (Figure S6) towards the product. The highest
point in this path is 3.6 kcal/mol above 3c, thus significantly below the 7.0 kcal/mol reported
above for the TS 3c-d' in the intramolecular WNA path.

10

Figure 4. Catalytic cycle for [(L3)CuII(OH)2] complex where both the intramolecular WNA and
intermolecular SET-WNA pathways are represented. Free energy changes for steps at the
electrode are indicated explicitly in Volts (green) and for steps in solution are indicated in
kcal/mol with respect to 3c (black).
The two catalytic pathways just described for the [(L3)CuII(OH)2] complex are summarized in
Figure 4. It is worth mentioning here that the low barrier obtained for the SET-WNA mechanism
in this 3c system is fully consistent with the reported turnover frequency of 100 s-1, that is the
highest described to date for Cu catalysts. More significantly, it confirms the prevalence of this
type of mechanism for Cu-based water oxidation catalysts, even when the ligand is not involved
in the redox process.

2. Extension to Ru systems? The case of [(L3)(damp)RuII(H2O)]2+

11

We finally explored how the SET-WNA mechanism could perform in cases where the
conventional WNA mechanisms are well established. We chose the [(L3)(damp)RuII(H2O)]2+
(damp= 2,6-bis((dimethylamino)-methyl)pyridine) system, hereafter [(L3)(L4)RuII(H2O)]2+, that
has been studied both experimentally and computationally.14

(4)
In eq. 4 we present the oxidation sequence that has been reported for this system. The active
species 4d can be viewed also as having a formal RuIV state, as there is a partial oxyl character
(0.4 e- in the oxygen and 0.6 in the Ru). A conventional WNA transition state has been
computationally reported for the interaction of this complex with an external water molecule
with a relative free energy of 20.7 kcal/mol. In this transition state, a lone electron pair of water
attacks the oxygen center on ruthenium, and leads to a hydroperoxyl intermediate,
[(L3)(L4)RuIII(OOH)]2+, that further evolves to generate dioxygen after one electron oxidation.

4f
33.5 (D)
33.4 (Q)
4f'

4d
0.0 (D)

4e
5.9 (D)

Figure 5. Free energy profile for the generation of RuIV-OH---HO• intermediate in
[(L3)(L4)RuII(H2O)]2+.
12

An eventual SET-WNA mechanism from 4d should go through an intermediate where an
electron is transferred from the external water to the complex. We found this process to be not
feasible and the results are summarized in Figure 5. An adduct between the complex and the
external water can be found, but the intermediate with the 2c-3e- bond between the two oxygen
atoms is not formed. We tried to force this type of species with transfer of a H• radical from
water to the catalyst (in a way analogous to the R-H activation chemistry previously reported by
Neese and co-workers).46 We could reach in this way the RuIV complex shown in Figure 4
(species 4f). This complex, either in doublet or quadruplet state, has an energy more than 10
kcal/mol above the competing WNA transition state, most likely because the oxygen centered
•OH radical is not sufficiently stabilized by the Ru-OH moiety. These results clearly indicate that
the SET-WNA mechanism is not operative in the Ru WOC chemistry.
An overview of the new mechanistic scenario

Figure 6. Overview of the water oxidation mechanisms. The oxygen directly attached to the
metal is marked in red, the incoming oxygen is marked in yellow.
13

The results reported above make a strong case that SET-WNA has to be added to the list of
mechanisms available for water oxidation. We make an effort in this section to put together all
available mechanisms in a single view, summarized in Figure 6. The overall process consists of
the abstraction of at least two electrons, either by chemical or electrochemical methods from a
metal complex, and the absorption by this complex of two electrons from two oxygen centers
(each in formal oxidation state -2) to make a peroxide bond (each oxygen atom in formal
oxidation state -1).
The left part of Figure 6 deals with the electron abstraction from the complex. The reaction
usually starts with the oxidation of the metal center in a reversible way. This step is shared by the
four systems analyzed in this work, with the peculiarity that for the Ru the two electrons are
mainly abstracted from the metal center. The next step involves the removal of an additional
electron that can occur at three different sites within the complex: the metal center, the ancillary
ligand or the oxygen ligand itself. In Ru complexes with neutral ligands such as
[(L3)(L4)RuII(H2O)]2+ described above, the electron is mainly abstracted from the metal center.
In the [(L3)CuII(OH)2] system, the second electron is removed from the oxygen ligand since the
IV/III metal based redox potential for copper is very high. This produces the corresponding
Cu(III)-oxyl species that is very active. Finally, the combination of a metal difficult to oxidize
and a redox-active ligand leads an intermediate situation with a ligand-centered radical, as
happens in [(L1)CuII]2- and [(L2)CuII(OH2)2].
After the oxidation is completed, the system is ready for the O-O bond formation step. Again,
three different mechanisms are available, water nucleophilic attack (WNA), single-electron
transfer water nucleophilic attack (SET-WNA) and interaction of two M-O units (I2M)
mechanism. The WNA mechanism is formally the simplest as it proceeds in one step through a

14

single transition state. A lone pair in the external oxygen attacks the metal-bound oxo center,
which in turn transfers two electrons to the metal. As a result, the metal oxidation state is
diminished by two units, and a single O-O bond is formed. The WNA mechanism requires thus a
metal center in a high oxidation state, which can remove enough charge from the oxygen center
to make it suitable for nucleophilic attack. This is most easily accomplished with second- or
third- row transition metal centers, as is the case for the [(L3)(L4)RuII(H2O)]2+ discussed above.
A second well-recognized mechanism is the I2M, which requires a relatively accessible oxyl
species that are able to couple. Each oxygen atom is already oxidized to oxidation state -1 by the
external oxidant and thus formally no metal reduction takes place. This mechanism requires
stable oxyl radical complexes with low charges, as the two complexes need to come together.
The additional mechanism we are proposing, SET-WNA, share similarities with the two
previously described but has some specific features. In SET-WNA, the external oxygen does not
transfer two electrons in a single step to the complex, but makes two single one electron
transfers. The most significant difference between SET-WNA mechanism and the WNA and
I2M just described is the formation of a (HO•••OH)- fragment, where there are two electrons in
the σ O-O orbital and one in the σ* O-O, thus with a bond order of 0.5. We have shown in the
examples above that it is operative for the three copper complexes studied.
The SET-WNA mechanism completes the general scheme by connecting the two conventional
mechanisms, as it may proceed in principle from either the M(n+2)+oxo systems typical of WNA
or from the M(n+1)+oxyl systems typical of I2M. More significantly, SET-WNA depends critically
on the stability of the M(n+1)+ (HO•••OH)- intermediate, defining thus a new paradigm for
catalyst optimization. The SET-WNA fits well with the characteristics of first row transition
metals, and has been shown to operate with fast catalysts where the overpotential is easily

15

controlled. The introduction of this new mechanism will help experimental and computational
chemists to design new efficient WOCs based on first row transition metals as well asexplore
and characterize their low energy path-ways.
Computational Details
All calculations were performed using the DFT approach with the B3LYP-D3 functional,48,49 and
a basis set with diffusion and polarization functions in all atoms. Solvation was introduced
implicitly with water as the solvent. The role of the introduction of a water molecule was
evaluated (Figure S7) and found to be not necessary. All geometry optimizations were carried
out in solution without symmetry restrictions. Unless otherwise mentioned, all reported energy
values are free energies in solution. Full computational details are provided in the Supporting
Information.
ASSOCIATED CONTENT
The supporting information includes extended computational details, additional mechanisms, and
Cartesian coordinates of all structures reported. A dataset collection of computational results is
available in the iochem-db database.50 This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* fmaseras@iciq.cat
* allobet@iciq.cat

16

Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Funding Sources
Any funds used to support the research of the manuscript should be placed here (per journal
style).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank MINECO (Grants CTQ2014-57761-R, CTQ-2013-49075-R, Severo Ochoa Excellence
Accreditation 2014-2018 SEV-2013-0319, CTQ-2014-52974-REDC) and the ICIQ Foun-dation.
I.F.-A thanks the Severo Ochoa predoctoral training fellowship (Ref: SVP-2014-0686662). P.G.B. thanks "La Caixa" foundation for a Ph.D. grant.
ABBREVIATIONS
6,6’-dhbp= [2,2'-bipyridine]-6,6'-bis(olate),
bpy= 2,2'-bipyridine
OPBAN= o-phenylenebis-(oxamidate)
damp= 2,6-bis((dimethylamino)-methyl)pyridine
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